METHOD FOR SCREENING POTENTIAL THERAPEUTICALLY EFFECTIVE ANTIVIRAL AGENTS

Abstract: The invention provides methods for screening agents for potential anti-viral effects by assessing the ability of the agents to suppress viral replication and/or pathology in thymic cells grown in thymic organ culture in vitro. Also provided are methods to study viral pathology and infectivity.

Description: METHOD FOR SCREENING POTENTIAL THERAPEUTICALLY EFFECTIVE ANTIVIRAL AGENTS DESCRIPTION Technical Field The present invention is directed to methods to screen potential therapeutic anti-viral agents, particularly those that suppress viral replication and/or pathology in thymus cells. Background Art The ability to obtain therapeutically effective anti-viral agents is facilitated by preclinical screening systems that are relevant, predictive and amenable to high throughput. Such systems include in vitro cell-free assays, in vitro tissue culture assays and assays in animal models. Important animal models have traditionally included laboratory mice, rats and non- human primates. More recently, specifically modified mice such as SCID-hu mice and hu-PBL mice have also played a role. However, even these higher-order models do not usually reflect human physiology in its entirety. Another drawback of animal testing is the need for extensive and direct contact with infected animals and, therefore, the potential exposure to drugs and viruses. This situation is particularly undesirable in the case of human immunodeficiency virus (HIV) . Furthermore, animal systems are usually time consuming and expensive, making them unsuitable for rapid screening of large numbers of potential therapeutic agents. For numerous viruses, there are no animal models; thus efficacy testing of putative therapeutic agents on human beings occurs only at the clinical stage. The extreme expense and safety concerns of this clinical testing pose a barrier to the current development of therapeutics and ultimate progress in delineating the mechanisms of viral infectivity and pathogenesis. Although n vitro drug screening systems exist, drug efficacy results from in vitro cell-free and cell culture systems do not always correlate well with in vivo efficacy. Nevertheless, in vitro systems are essential in modern pharmaceutics for large-scale screening of agents with therapeutic potential. The more closely an in vitro system mimics in vivo infection, the more useful the in vitro assay. Thus, in vitro drug screening assays are constantly being developed to more closely mimic in vivo responses. HIV is the name given to a group of highly related viruses which has been identified as the primary etiological agent of the acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC) in humans. HIV is a deadly infectious disease and a major worldwide health problem. The most salient feature of HIV infection during the progression towards AIDS is the time-dependent loss of CD4 + T cells. Many mechanisms have been postulated to underlie such a process, including direct killing of infected cells by the virus itself, killing of infected and uninfected CD4 + T cells by cytolytic T cells directed against viral antigens bound to target cells surfaces, and antibody-directed cell killing. Few antiviral therapies, if any, result in the resurgence of predepletion levels of circulating CD4 + T cells during the course of treatment. There are several explanations for this observation. First, the therapies may be ineffective in blocking de novo viral infection and/or replication. Second, and perhaps equally important, is the possibility that the regenerative capacity of the immune systems has been abrogated by the pre-existing HIV infection and antiviral therapy is incapable of restoring this function. Recently, there has been an increased focus towards understanding the mechanisms of HIV pathogenesis in the context of human lymphoid organs. It is in these sites that much of hematolymphoid development, activation, and function take place. Moreover, it has been suggested that various lymphoid organs might serve as reservoirs for HIV and consequently may be the major sites where HIV mediates its deleterious effects. However, progress towards understanding the pathogenesis of HIV infection in vivo and discovering therapeutic agents has been hampered by a lack of a cost-effective, efficient, and clinically relevant in vitro screening system. One of the most widely-used animal models for HIV infection is the rhesus macaque and infection with simian immunodeficiency virus (SIV) . In this system, large amounts of drugs must be synthesized and isolated prior to screening, an expensive and time consuming task. Maintaining the animals is expensive. Because of the high cost of the monkey model, it is only used to test drugs already showing strong indications of success in in vitro testing. Moreover, the monkey model utilizes a different, although related, virus, and it is not necessarily true that antiviral compounds active against SIV will be active against HIV, and vice versa. What is needed to develop anti-viral agents is an effective in vitro screening system that reflects in vivo pathogenesis and which is capable of rapidly screening a wide variety of agents. Such an in vitro system should provide the reliability of the more expensive in vivo systems and yet afford rapid, cost- effective initial screening for potential therapeutics from a virtually limitless variety of sources. To the extent that such a system is relevant to HIV infection in vivo, it should also be useful for the evaluation of pathogenic mechanisms associated with HIV infection. Disclosure of Invention The present invention is directed to a method of screening agents for therapeutic potential. The method comprises incubating thymic lobules, preferably human fetal thymic lobules, for a time and under conditions sufficient to allow viral infection. The lobules are then exposed to a virus, and, either prior to virus exposure, simultaneously, or shortly thereafter, exposed to an agent to be screened. The lobule contents are then assayed for suppression of viral infection. Therapeutic potential is present if any indicia of viral infection are diminished relative to a control sample that has not been exposed to the agent. The method can also be used to study viral transmission, infection, replication, and pathogenesis. Definitions The following definitions refer to terms and phrases used herein. Other definitions appear as indicated. Suppression indicates that there is evidence of reduced viral transmission, infection, replication, or pathology. Assaying cells for viral activity is done by observing any factors such as cell viability and yields and CD4 and CD8 phenotypes. A control sample is one in which thymic lobules are obtained and incubated under identical conditions as the lobules to be infected but not exposed to the virus to be tested. Pathology is viral infection, replication, cell death, slowing or aberrant cell growth or atypical cellular phenotype or function due to viral infection. An anti-viral agent is one that prevents or treats viral infection, i.e., one that can be used prophylactically or therapeutically. Such an anti-viral agent can affect any of the mechanisms of virus transmission, infection, replication, or pathology. A potential anti-viral agent is an agent which has been shown to suppress viral infection and/or pathology in the method described herein. Such a potential anti-viral agent has an enhanced likelihood of being effective in human prophylaxis and therapy. Potential anti-viral agents can come from a virtually limitless variety of sources. Viral activity refers to viral infection, re¬ infection, replication, expression of viral proteins or pathology. Brief Description of Drawings Figure 1 is a graph depicting viable cell yields per incubated thymus fragment in uninfected cultures. Figure 1 is discussed in Example 1. Figure 2 contains two graphs depicting relative and absolute numbers of CD4 + /8 + cells in uninfected cultures. Figure 2 is discussed in Example 1. Figure 3 is a graph depicting cell viability of cultures infected with HIV by the standard drip or dip method. Figure 3 is discussed in Example 3. Figure 4 is a graph depicting p24 levels in cultures infected with HIV by the standard drip or dip method. Figure 4 is discussed in Example 3. Figure 5 contains two graphs depicting the relative (a) and absolute (b) numbers of CD4 + /8 + cells in cultures infected with HIV by the standard drip or dip method. Figure 5 is discussed in Example 3. Figure 6 is a graph depicting the number of cells in cultures infected with HIV by the standard drip or dip method. Figure 6 is discussed in Example 3. Figure 7 is a graph depicting the relative number of CD8 + cells in cultures that have been infected with HIV by the standard drip or dip method. Figure 7 is discussed in Example 3. Figure 8 is a FACS forward scatter, side scatter, CD4 and CD8 expression patterns following "rocking" infection in TOC (day 8) . Note: HXB2 infection is from a separate experiment, and is included for comparison only. Figure 8 is discussed in Example 3. Figure 9 is a graph depicting p24 levels of HIV-infected cultures over time. Figure 9 is discussed in Example 4. Figure 10 contains two graphs depicting viable cell yields per thymus fragment in HIV-infected cultures. Figure 10 is discussed in Example 4. Figure 11 is a series of graphs depicting relative and absolute numbers of viable CD4 + /8 + cells in HIV infected cultures. Figure 11 is discussed in Example 4. Figure 12 is a series of graphs depicting fluorescence activated cell so ter (FACS) analysis of HIV-infected cultures. Figure 12 is discussed in Example 4. Figure 13 is a summary of p24 values over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as geometric mean values including standard error bars. Negative control values were zero at all time points and this data is not shown. Figure 13 is discussed in Example 4. Figure 14 is a summary of cell yields per fragment over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as mean values including standard error bars. Figure 14 is discussed in Example 4. Figure 15 is a summary of percent live cells recovered per fragment over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as mean values including standard error bars. Figure 15 is discussed in Example 4. Figure 16 is a summary of CD4 + CD8 + profile over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as mean values including standard error bars. Figure 16 is discussed in Example 4. Figure 17 is a summary of CD4:CD8 ratios over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as mean values including standard error bars. Figure 17 is discussed in Example 4. Figure 18 is a summary of percent CD8 + thymocytes over time following infection in TOC with the molecular clone of HIV, NL4.3. Data is presented as mean values including standard error bars. Figure 18 is discussed in Example 4. Figure 19 is a graph depicting p24 levels in HIV-infected cultures treated with azidothymidine (AZT) or dideoxyinosine (ddl) . Figure 19 is discussed in Example 6. Figure 20 contains two graphs depicting relative and absolute numbers of viable CD4 + /8 + cells in HIV-infected cultures treated with AZT or ddl. Figure 20 is discussed in Example 6. Figure 21 is a graph depicting relative numbers of CD8 + cells in HIV-infected cultures treated with AZT or ddl. Figure 21 is discussed in Example 6. Figure 22 is a graph depicting p24 levels in HIV-infected cultures (EW strain) treated with AZT or ddl. Figure 22 is discussed in Example 6. Figure 23 is a graph depicting p24 levels in HIV-infected cultures (NL4-3 strain) treated with AZT or ddl. Figure 23 is discussed in Example 6. Figure 24 shows the effect of the protein tyrosine kinase inhibitor, herbimycin A, on Class I (W632) expression and TdT reactivity (TdT-FACS) following infection with HIV (JD) in TOC. In Figure (a) , Class I expression is upregulated after infection and levels of upregulation are not affected by addition of herbimycin. In Figure (b) , TdT reactivity increases with infection in the absence of drug. As drug dose increases, TdT reactivity is reduced to normal and subnormal levels. In Figure (c) , drug toxicity can be seen by day 7 at high doses for negative controls, and for infected groups, protection is lost. Note: All plots are gated on live lymphoid cells (based on forward and side scatter profiles) and cells expressing both CD4 and CD8 (double positives). Figure 24 is discussed in Example 6. Figure 25 shows the structure of cyclin Bl. Figure 25 is discussed in Example 7. Figure 26(a) shows Anti-Cyclin Bl Western analysis of cytoplasmic lysate from day 4 uninfected and JD-infected TOC thymocytes, while Figure 26(b) shows a Western blot of lysates from Scid-hu thymocytes probed with mAb anti-Cyclin Bl (GNS1) . Figure 26 is discussed in Example 7. Modes for Carrying Out the Invention The present invention is directed to methods of screening agents for potential anti-viral efficacy and elucidating the mechanisms of viral infection, transmission, and pathogenesis. It has now been found that HIV infection of cultures of intact human thymic lobules mimics the behavior of existing animal models for HIV infection, including the SCID-hu Thy/Liv mouse model as described in Bonyhadi et al.. Nature, 363:728-732 (1993). The system is also known as TOC for thymic organ culture. The TOC system is useful for rapid screening of compounds for anti-viral potential, particularly those for the treatment and prevention of AIDS. The TOC system is much simpler, safer, faster, and more cost effective than existing animal models. The TOC system is also more predictive of the actual efficacy of known anti-HIV compounds in humans than other in vitro systems. Utilizing the TOC method described herein, known therapeutic agents have been found to exert therapeutic efficacy similar to that found in vivo. More specifically, both 2' ,3 , -dideoxyinosine (ddl) and azidothymidine (AZT; also referred to as zidovudine) suppressed HIV replication and pathology at equivalent dose ranges found to be active in humans. In fact, the thymic lobule culture system parallels the SCID-hu in vivo system, but demonstrates faster kinetics. Thus, the results from initial screening of compounds provided by this invention are more indicative of actual results in human clinical trials than results from previously- described in vitro systems. In one embodiment, a method is provided to screen agents for potential anti-viral efficacy. The method comprises providing thymic lobules; incubating the lobules for a time and under conditions sufficient to maintain cell viability and to allow viral infection; exposing the lobules to a virus under conditions to induce infection; exposing the lobules to an anti-viral agent to be screened; and assaying for evidence of viral activity. The agent is a potential anti-viral agent if it suppresses viral pathology relative to lobules incubated in a control sample. The lobules obtained are thymic. Preferably, the thymic lobules are human, including but not limited to neonate and mature human. More preferably, the thymic lobules are from human fetal thymus tissues. The lobule is a relatively intact lymphoid organ subunit that contains many cell types such as thymocytes, macrophages, dendritic cells, and epithelial cells. Thus, it provides a more complex and in vivo-like environment than simpler cell cultures. Thymic lobules are obtained by dissecting the whole thymus into thymic fragments. Each fragment contains at least one intact thymic lobule. The fragment is incubated separately under conditions that support cell viability and allow infection of the lobules by the virus. Suitable methods of cell culture known in the art can be used. In practice, a fragment is placed on top of a sterile filter laying across an absorbable gelatin sponge boat in a tissue culture plate containing support media. Thymic lobules may be used fresh or frozen and thawed before use. Preferably, fresh thymic fragments are used. Standard cell culture techniques are adequate for use in the methods described herein. Culture plates are maintained in incubation chambers at 37°C in an atmosphere of 5% C0 2 . Alternatively, a high oxygen culturing environment, i.e. an environment containing about 40% to 60 % oxygen, preferably about 50% oxygen, is effective in reducing central necrosis that may occur in the cultured fragments. A precision oxygen regulator device may be used to maintain this high oxygen environment. Typically, this type of device attaches to the standard tissue culture incubators and is used to maintain the oxygen content in the incubator. Incubation periods prior to infection may range from essentially no incubation prior to infection to periods as long as the lobules can be maintained in a viable state and still retain the ability to be infected by the target virus. Typically, incubation periods range from approximately less than one day to about fourteen days. Although the optimal medium varies depending on the lobule source, the type of virus, and the length of the incubation period, the preferred medium is one which supports cell viability and allows infection of the lobules by the target virus. Preferably, the medium is Roswell Park Memorial Institute medium (RPMI-1640) supplemented with 10% fetal calf serum (FCS) , glutamine, penicillin and streptomycin. Successful maintenance of cell viability is dependent on maintenance of intact lobules and an air interface for cultured lobules. Preferably, medium is changed at least every two days. More preferably, it is changed daily. Preferably, IX I/S/T (insulin/selenium/transferrin, Sigma) and vitamin supplements (GIBCO MEM additive) are added to the medium to enhance cell viability. The methods described herein are suitable for use in screening potential therapeutic agents against a wide variety of viruses. Candidate viruses would include those which infect cells found in the thymus. Such viruses include but are not limited to human cytomegalovirus, human immunodeficiency virus (HIV) , herpes simplex virus (HSV) , rubella virus, hepatitis virus, measles virus and varicella zoster virus (VZV) . The cells found in the thymus include but are not limited to thymocytes, dendritic cells, macrophages, endothelial cells, fibroblasts, B cells, and epithelial cells. Preferably, the viruses are those which infect thymocytes or cells in the thymic stro a which are important for T cell development. More preferably, the viruses are various isolates of HIV. Suitable isolates of HIV are those which are most similar to those which infect human patients. Such isolates include but are not limited to primary isolates such as EW and JD and infectious molecular clones such as JR-CSF, Xho, and NL4-3. The types of anti-viral agents suitable for testing in this assay are virtually unlimited. Agents can be obtained from chemical, nutritional and biological sources. For instance, suitable chemical agents can be novel, untested classes of chemicals, as well as agonists, antagonists, or modifications of known therapeutic agents. Nutritional agents can be simple or complex, ranging from minerals to extracts from plant and animal sources. Such agents can be easily derived from plant sources including but not limited to soy, pea, and potato by-products, and from animal products such as whey or other milk byproducts. Biological agents include but are not limited to biological response modifiers (such as cytokines) , antibodies, and other so-called "small molecules." Such agents can either be derived from biological sources or chemically synthesized to mimic the effect of biological response modifiers. Anti-viral agents may also include those mediating an effect through a cellular vehicle. For example, infected, uninfected, transduced, or non-transduced cells of various phenotypes (such as monocytes, macrophages, epithelial cells) may be introduced, such as by microinjection, into the lobules to assess their effect on viral replication or pathology. The assay allows rapid screening of potential anti-viral agents, thus enabling the screening of large numbers of different agents at multiple concentrations. The therapeutic agents tested to date have been effective in this assay at concentrations in the sub-micromolar range, indicating that the assay is extremely sensitive. This is a real benefit over current animal systems which require large amounts of agents for screening. Preferably, the range of concentrations of agents used in this assay is about 0.001 μM to 5 mM. The upper range is limited by the solubility of the agent and toxicity of high concentrations of some drugs; the lower range is not necessarily limited but should be low enough to encompass a complete dose-response curve. The lobules can be infected by any method known in the art, including, but not limited to both the "drip" and "dip" methods. In the standard "drip" method, virus is added to the lobules by dripping virus-containing liquid over the top of thymic lobules being incubated on absorbable gelatin sponge boats. Preferably, however, the lobules are infected using the "dip" method. In the dip method, the lobules are incubated in a liquid containing the virus particles for a length of time suitable to allow infection. In a preferred variation of the dip method, the lobules are subjected to gentle rocking or rotation every 5 to 30 seconds during the period they are. immersed in the liquid. Preferably, they are immersed in the liquid for about 2 hours. The amount of virus used for infection (reported as infectious dose) depends on the type of virus, the method of infection, the incubation conditions of the cells, etc. The infectious dose necessary to follow the methods herein is determined empirically. Such determination is well within the skill of one in the art. A suitable infectious dose is one which allows measurement of a parameter of viral activity and/or infectivity. In the examples described below, HIV is obtained from phytohe agglutinin (PHA)-stimulated blast cells previously infected by HIV. The "undiluted" virus is simply the supernatant derived from those cells. Infection by dipping the cells in undiluted virus appears to accelerate infection and the onset of observable HIV pathology. The anti-viral agents to be assayed are introduced into the lobules prior to, simultaneously with, shortly after or several days after viral infection. The time of introduction will vary according to the type of virus, the incubation conditions and the method of infection. It may be useful to test the same agent at a variety of time points to insure accurate determination of efficacy. The optimal conditions in the case of HIV are to pretreat the lobules with various agents approximately 1-2 hours before infection and to incubate the pre-treated, infected lobules in presence of the agent. After addition of the agent, the lobules are further incubated for a time sufficient to allow an assayable parameter of virus infection or pathology to occur in the control cells. In other instances, agents may be added several days post-infection. Tissue fragments are harvested and disrupted to allow the release of thymocytes and other lobule contents. Extent of viral infection is determined by assaying any suitable parameter indicative of viral replication or viral-induced pathology. These parameters include but are not limited to: viral-specific DNA, RNA, or protein production, viral-induced cytokine production, thymocyte phenotype profile as determined by FACS analysis, histological analyses, apoptotic profile, DNA quantitation, DNA fragmentation profiles, immunohisto- chemistry, functional assays, PCR analysis of integrated proviral DNA and 2-LTR circular viral DNA, and isolation of virus in vitro. A positive response to the anti-viral agents is manifest when cells from the lobules treated with the agent exhibit a suppressed level of viral pathology and/or replication compared to infected, untreated controls. The extent of viral replication or pathology is determined by measuring one or more of the various parameters above. In another embodiment, the present invention is suitable for testing hypotheses concerning mechanisms of viral infection, replication, and pathology in thymic cells. Incubated thymic lobules are infected with the virus as discussed above. Lobules can be harvested at various times after infection, and a variety of viral infection parameters can be observed. Alternatively, the virus used for infection can be specifically altered genetically, and the viral activity and replication compared to infection with native, non-altered control viruses. Use of drugs or other agents with known mechanistic targets can delineate new elements of viral pathology, thereby providing new targets for intervention through cellular therapy, gene therapy, drug therapy, or rational drug design. All documents cited herein are hereby incorporated by reference. The following examples are provided to illustrate but not limit the claimed invention. Examples Example 1 Cell Culture Technioues Fresh human fetal thymus was obtained at 18-24 weeks gestational age and kept in sterile media at 4°C. Human fetal tissues can be obtained with informed consent from agencies such as International Institute for the Advancement of Medicine (Exton, PA) or Advance Bioscience Resource (Alameda, CA) , in compliance with regulations issued by State and Federal governments. Fragments of approximately 1 mm 3 were dissected from the fresh thymus. These fragments represent approximately 1-4 intact thymic lobules as judged by light microscopy. Fragments are suitable for this test if they are big enough to contain intact lobules and small enough to allow diffusion of virus and agents. Typically this is about 1 mm 3 . Fragments were placed on sterile CoStar Nucleopore® filters (13mm/0.8μm) that lie across Gelfoam® sterile sponge boats (Upjohn) saturated in growth media. Growth media was RPMI (1640) with 10% FCS, glutamine, penicillin and streptomycin. Culture plates, typically Cell Wells™, 6-well flat bottom 35mm polystyrene plates (Corning) , were maintained in incubation chambers at 37°C under 5% C0 2 . Preferably, the culture medium was enriched every 24-48 hours by removing old medium and then dripping fresh medium over fragments. Incubation periods ranged from less than one day up to approximately 2 weeks. Typically, a majority of the cells of the lobules were viable for approximately 10 days under these culture conditions. The number of viable thymocytes from the lobules over a two week incubation period is shown in Figure 1. The results show that a significant loss of thymocytes occurred during culture. Relative and absolute CD4 + /8 + profiles of the cells from the thymic lobule culture are shown in Figures 2(a) and 2(b), respectively. These figures showed a depletion in this cell population over the two-week incubation period, indicating a significant baseline of cell loss/death in this culture system. Since viral infection can also cause cell loss/death, it was desirable to develop culture conditions that maximized thymus fragment cellularity and cell viability. Two experiments were performed in order to reduce the background cell death observed in the TOC system. In the first experiment, various modifications were made to the media. In a second experiment, the TOC were incubated in a relatively high oxygen content. The details and results of these two optimizing experiments follow in parts (a) and (b) below, a. Media Modifications Various media modifications were tested for their ability to enhance overall thymus fragment stability. The culture conditions were modified as follows: a) variation in FCS concentrations (2-20%) ; b) addition of the reducing agent 2- mercaptoethanol (2-ME to a final concentration of 50 μM) ; c) addition of epidermal growth factor (EGF at 10 ng/ml) ; d) addition of IX supplement containing trace amounts of I/S/T (Sigma) . In the case of 2-ME, EGF, or I/S/T containing media, 10% FCS was used. Uninfected cultures were assayed on day 11 for cell yields, percent viability, and CD4 + /8 + distribution patterns. The results obtained are shown in Table 1. Data represent mean values averaged from three fragments cultured under identical conditions for 11 days. These data suggest that supplementing media with IX I/S/T helped maintain thymocyte longevity in culture, as evidenced by viability, increased representation of CD4 + /8 + thymocytes and increased cell yields following 11 days culture. Varying other culture conditions did not produce any obvious growth or maintenance advantages. Table 1 Effect of cell culture conditions on thymic cell viability Cells per % % % % Absolute Culture Fragment Live CD4 + CD4- CD4 + Number Condition (xlO 3 ) Cells 8- 8 + 8 + CD4 + 8 + 2% FCS 113 27 39 13 39 43000 10% FCS 122 32 31 14 46 59000 20% FCS 83 25 38 12 40 34000 2-ME 99 27 47 11 32 34000 EGF 43 17 35 9 45 20000 IX I/S/T 167 43 25 12 56 96000 The effects of different lots of FCS, vitamin supplements (GIBCO MEM additive) , addition of 2-ME (50 μM) to I/S/T, and changing the growth medium every other day (qod) instead of daily (qd) were observed and are reported in Table 2. Table 2 Effect of cell culture conditions on thymic cell viability Cells % Culture Per Live % % % Condition Fragment Cells CD4 + 8- CD4-8 + CD4 + 8 + (x 10 3 ) FCS#1 166 26 27 9 37 FCS#2 130 33 35 14 43 FCS#3 194 22 43 11 41 FCS#1 108 24 44 10 37 +2-me FCS#1 116 44 48 12 35 +vitamins FCS#1 144 12 38 9 46 qod In Table 2, data represent mean values averaged from five fragments cultured under identical conditions for 12 days. Cultures were assayed on day 12 for cell yields, percent viability, and CD4 + /8 + distribution patterns. Different lots of FCS had little or no effect on thymus cell viability, and the addition of 2-ME to I/S/T-supplemented medium offered no apparent advantage to culture maintenance. Vitamin supplements appeared to confer some benefits in overall cell viability at day 12. Cultures fed every other day with I/S/T-supplemented medium had fewer viable cells than cultures fed every day. Based upon these results, it is preferred that thymic lobules be maintained with I/S/T-supplemented medium that is replaced daily. b. Oxygen modifications: Eighteen fragments obtained as described in Example 1 were placed on filters and sponge boats and incubated in the culture media as described above. Eight of these fragments were separately incubated for eight days under normal conditions, i.e. 37°C, 5% C0 2 , 95% air (which corresponds to about 20% oxygen) . The remaining ten fragments were also incubated at 37°C but in a environment containing about 50% oxygen and 5% C0 2 . The results of this experiment are shown below in Table 3: ω NJ NJ H o U) O Ul Ul Table 3 Comparison of normal vs. high (50%) oxygen culture conditions in TOC. Data is presented as mean values from 8 to 10 fragments for each condition. Oxygen number of Cells Percent CD4:CD8 Percent Percent Percent Days in culture level fragments x 10 3 Live Gate ratio CD4 + CD8 + CD4 + 8 + 8 normal 8 218 (±33 62 1.2 9 8 82 s.d.) 8 high 10 502 (±8 72 1.3 12 10 76 s.d.) As can be seen from Table 3, the presence of 50 % oxygen results in improved cell recovery per fragment. This indicates that the high oxygen content appears to reduce the amount of central necrosis that has been observed between days 3 and 5 in culture. Histological examination of fragments show a nearly complete ablation of central necrosis under high 0 2 conditions. Example 2 Methods of Obtaining HIV for Infection of Thymic Lobules PHA-activated T-cell blasts are required for preparing the HIV virus stocks. PHA blasts were either freshly prepared or frozen. Frozen cells must be thawed and put into culture media containing interleukin-2 (IL-2) at least 24 hr before HIV infection. Fresh cells are preferable to frozen cells. PHA-activated human T-cell blasts were produced by obtaining buffy coats of human cells from a local blood bank. Cells were removed from the bag under sterile conditions and mixed with an equal volume of phosphate-buffered saline (PBS) or Hank's balanced salt solution (HBSS) with heparin (2 ml of 1000 units/ml made up in 250 ml of PBS or HBSS) . Approximately 35 ml of the diluted cell suspension was aliquoted into 50 ml conical centrifuge tubes. The cell suspension was underlayered with approximately 14 ml of Ficoll-hypaque (HISTOPAQUE®, Sigma) and centrifuged at 450 RCF (about 1400 rpm) for 30 min with the brake off. The interface cells were harvested, washed twice with PBS and counted. In some cases, the cells were resuspended in 50 ml, diluted 1:5 and counted. For incubation, cells were resuspended at 2-3 x 10 6 per ml in Iscove's Modified Dulbecco's (IMDM) media containing 10% FCS, 2 mM L-glutamine, penicillin to a final concentration of 100 U/ml, streptomycin to a final concentration of 100 μg/ml and 1 μg/ml of phytohemagglutinin-P (PHA-P) . Cells were incubated, maintaining a cell density of 2-3 x IO 6 for three days and either frozen in 10% dimethyl sulfoxide (DMSO)/FCS and stored in liquid nitrogen, or used directly in infection experiments. When frozen PHA blasts were used, the PHA blasts were thawed and incubated overnight in medium containing 50 units/ml of IL-2 prior to infection. PHA blasts were then pelleted by centrifugation at 1000 rpm for 5 min. The PHA blasts were infected by resuspending the blasts in virus (10 3 -10 4 TCID 50 ) containing 5 μg/ml of polybrene at 1-2 x 10 7 cells/ml of virus. Viruses for infection must be thawed quickly at 37°C and used as soon as possible. The starting concentration of virus inoculum was 5 ml of a stock with TCID 50 of 10 3 or IO 4 per 10 8 cells. Infection was allowed to occur for about 2 hr at 37°C. After two hours, cells were diluted up to the appropriate volume in medium containing IL-2 at 50 Units/ml at a cell density of 2-3 x 10 6 /ml. On day 2 or 3 (depending on virus) , the cells were pelleted and fresh medium was added. Starting on day 3 or 4, supernatant was collected until day 8 or 9 at 24-hr intervals, and fresh medium containing IL-2 was added. The collected supernatant was aliquoted into freezing vials labelled with the date and type of virus. An aliquot also was saved for p24 analysis by adding 1/10 volume of 10% Triton X-100 to the supernatant and storing at 4°C until analysis. The freezing vials containing virus were frozen immediately in liquid nitrogen. Example 3 Infection Techniques The following is a brief description of the viruses used in these studies: 1. NL4.3 is an HIV clone which demonstrates T tropism in vitro, grows well on PHA-stimulated PBL blasts, and causes rapid thymocyte depletion in SCID-hu Thy/Liv mice concomitant with a rapid increase in viral p24 antigen levels. 2. XHO is an HIV clone consisting of a NL4.3 backbone whose 3' nef-encoding region and most of the 3' LTR have been replaced by corresponding sequences from the macrophage tropic virus JR-FL. XHO grows well on PHA-stimulated PBL blasts and demonstrates NL4.3-like pathology in infected SCID-hu Thy/Liv mice. 3. JR-CSF is an HIV clone which grows well in PHA-stimulated PBL blasts, but replicates slower than XHO and NL4.3 in SCID-hu Thy/Liv mice. Additionally, it requires longer periods of infection before thymocyte deletion is observed. 4. RIP7 is derived from the HIV molecular clone HXB2 which has had its vpr. vpu and nef encoding regions deleted. RIP7 grows fairly well in PHA- stimulated PBL blasts and very well in cell lines, but does not appear to be infectious in SCID-hu Thy/Liv mice. 5. EW is a primary isolate of HIV-1 which grows well in PHA-stimulated PBL blasts and causes thymocyte depletion in SCID-hu Thy/Liv mice, albeit more slowly than NL4.3 and XHO and more rapidly than JR-CSF. 6. SM was isolated from a patient classified as having symptomatic HIV infection and not on any antiviral drug regimen. SM is cytopathic for PHA- stimulated PBL blasts. a. Comparison of the "Drip" versus "Dip" Method The "standard" method of infection of thymic organ cultures involves dripping virus-containing PHA blast supernatant over the top of the organ fragment, the so-called "drip" method. In the case of thymic lobules already deposited on Gelfoam® boats as described in Example 1, the effectiveness of this method was determined. To increase the initial infectious dose at time zero, an alternative infection protocol, the "dip" method, was developed. In the dip method, thymic lobules were incubated for approximately 2 hours in vials of undiluted virus-containing PHA blast supernatant. Dipped fragments were subsequently transferred to Gelfoam® boats for incubation. For infectivity analysis, tissue fragments were harvested and disrupted using tissue homogenizers which allow for the release of lobule contents (thymocytes and other thymus constituents) . Cells were counted and processed by: staining for flow cytometric analysis, quantitation of DNA content, immunohistochemistry, functional assays, and assessment of viral replication.. The results of these experiments are summarized in Figures 3-7. In general, here and as described in the HIV-infected SCID-hu mouse, inoculation of TOC with HIV (Xho in this example) resulted in decreased cell viability (Fig. 3), detectable p24 levels (Fig. 4), and decreased numbers of CD4 + CD8 + thymocytes (Figures 5a and b) . Dip-infected TOC followed approximately the same time course of HIV (Xho)-induced cell depletion as drip- infected cells and displayed lower viability by day 11 post-infection, as shown in Figure 3. Early in the infection, dip-infected cells produce more p24 than drip- infected cells; by 11 days post-infection, however, p24 levels were the same for cells from lobules infected by either method (Figure 4) . HIV p24 antigen was determined using a commercially available p24 ELISA kit (DuPont) according to the manufacturer's instructions. The results are presented as pg/10 6 cells. Dip-infected TOC showed a greater decrease in the CD4 + 8 + population (Figures 5a and b) and displayed a lower cellularity (Figure 6) than drip-infected lobules. The relative population of CD8 + cells was higher in dip- infected lobules than in drip-infected lobules, as shown in Figure 7. Thus, the dip method of infection appeared to increase the initial infectious dose and to accelerate the spread of infection and the onset of pathology compared to the drip method. b. Modified "Dip" Method A modification of the "dip" method involves gentle rocking of the thymus fragments in undiluted virus stocks. The rationale behind this approach is that with the fragments moving continually throughout the virus- containing medium, a higher multiplicity of infection can be reached than would be achieved with fragments remaining immobile at the bottom of the incubation vessel. In general, infection with TOC with pathogenic variants of HIV, such as NL4.3, EW and JR-CSF require infections with virus titers of around 10 3 TCID 50 in order to observe distinct signs of pathology by day 8 post- infection. In this experiment, virus stocks having titers in the IO 2 TCID 50 range were tested using this modified "rocking-dip" method. More specifically, TOCs were infected with JR-CSF, NL4.3, EW, or JD, where all but JR-CSF had titers of about IO 2 TCID 50 . (JR-CSF was estimated to be about IO 3 TCID 50 at the beginning of 1993.) The TOCs were incubated with the various viruses for two hours with continual rocking and then cultured for 8 days. Thymocytes were isolated at day 8 and analyzed for p24 expression. The results of this experiment are shown in Table 4 and Figure 8. co N) NJ H Ul o Ul O Ul O Ul Table 4 Comparison of p24 values and FACS profiles in TOC following infection via the rocking method with different isolates of HIV. Data is represented as mean values as averaged from 6 to 7 fragments cultured under identical conditions. Cells per p24 pg/10 6 days post- # of fragment % cells CD4:8 percent percent percent infection HIV fragments x IO 3 Live ratio CD4 + CD8 + 4+8 + 8 - 6 227 43 0 2.04 9.75 4.75 83 8 JR-CSF 7 210 10 12830 2.12 17.65 8.69 68 8 NL4.3 7 35 11 12605 0.86 9.01 10.65 71 8 EW 7 44 7 31701 0.64 12.25 19.11 55 8 JD 6 39 1 58173 0.24 8.16 37.44 24 As can be seen from these data, infections using the "rocking-dip" method with lower than normal virus titers are still sufficient to induce, by day 8 post-infection, the typical deletion profiles associated with HIV- mediated pathogenesis seen in human fetal thymus. This ability to use "low" titer stocks of virus should reduce the amount of time needed to make "effective" virus stocks and will allow for easier examination of HIV strains which typically do not yield high titer stocks. Example 4 Pathology of HIV in the System This example shows that infection in TOC generates a pattern of HIV-induced pathology similar to that observed following HIV infection in SCID-hu Thy/Liv mice. In one experiment, a SM isolate of HIV was used to infect the TOCs using the drip method as described in Example 3a. The results from this experiment are shown in Figures 9 through 12. In a second experiment, the TOCs were infected with the HIV molecular clone NL4.3 using the dip method. The results from this experiment are shown in Figure 13-18. In each experiment, the cells from lobules were harvested at varying times post- infection. As can be seen from Figures 9 and 13, the p24 production of the infected TOCs increased significantly over the uninfected controls. For the SM-infected TOCs, increased cell death and decreased cellularity compared to uninfected controls is shown in Figures 10(a) and 10(b) respectively. Similarly, Figures 14 and 15 show decreased cell numbers per fragment and decreased percentages of live cells per fragment for the NL4.3 infected TOCs. Cell death and decreased cellularity are determined by decreased representation of cells with a high forward scatter/low side scatter profile on FACS analysis. Time-dependent depletion in the relative and absolute number of CD4 + 8 + thymocytes is shown in Figures 11(a) and (b) for the SM-infected TOCs. Figures 11(c) and 11(d) show that the percentage of CD4 + 8 + cells per fragment and the absolute number of CD4 + 8 + cells per fragment, respectively, decreased over time for the SM- infected TOCs. FACS analyses of SM-infected cells from the lobules are shown in Figure 12. HIV (Xho strain) infected thymic lobules (lower panel) had a lower cell viability (left panel) (as determined by decreased representation of cells with a high forward scatter/low side scatter profile, typical of live cells) and CD4 + 8 + population than uninfected controls (upper panels) . Similarly, Figure 16 shows a decreased percentage of CD4 + 8 + cells for the NL4.3 infected TOCs. Additionally, Figure 17 shows that the CD4:CD8 ratio decreases over time following infection in TOC with the NL4.3 clone while Figure 18 shows that the percentage of CD8 + thymocytes increases over time. In summary, these results indicate that the thymic lobule culture system displayed parameters associated with viral infection. In addition, these results show that the differences between control groups and infected groups in the TOC system, using either the drip or dip methods of infection, can be seen as early a day 5 post-infection. This allows for a more rapid induction of pathology than is detectable in the SCID-hu Thy/Liv model. This also suggests that the TOC system may serve as a reliable pre-screen system prior to, or concurrent with, analysis in the SCID-hu Thy/Liv system. Example 5 Comparison of Infectivity Parameters of Different Strains of HIV To determine if infection of the thymic organ system is reflective of in vivo infectivity, Xho, NL4-3, EW, and RIP7 HIV isolates were tested. The isolates were obtained as described in Example 2. Infection was carried out by the dip method described in Example 3. The results are shown in Table 5. Data are mean values averaged from 6 fragments cultured under identical conditions for 8 days. The pathogenic effects of the Xho, NL4-3, and EW HIV isolates were similar, indicating that the thymic lobule culture system can be used for a variety of viral strains, thus expanding its applicability. In contrast, the RIP7 strain, which grows well in PHA blasts j-n vitro and does not appear to replicate in SCID-hu Thy/Liv mice, did not grow well in the thymic lobule culture system. These data, taken together, suggest that the thymic lobule culture system more closely reflects the j-n vivo SCID-hu mouse system. Example 6 Assay of Therapeutic Agents To determine whether the cell culture system described in Example 1 was effective as an jji vitro model for screening potential anti-viral agents, known therapeutic agents were tested for their ability to inhibit viral infection. In a first series of experiments, the virus used was HIV strain Xho. The known drugs were azidothymidine (AZT) and 2',3 , -dideoxyinosine (ddl) , both of which suppress HIV infection in humans. In a second series of experiments, the virus used was JD, a primary isolate of HIV, and the drug tested was a variety of protein inhibitors, a. Effects of Reverse Transcriptase Inhibitors (AZT and ddl) : Varying amounts of AZT or ddl were added to lobules which had been infected with the HIV Xho strain by the dip method as described in Example 3. In this experiment, the growth medium contained no I/T/S supplement, as described in Example 1. Lobules were harvested at various days post-infection and their cellular contents were assayed for various parameters of infection and pathology. One culture of control lobules was infected but received no therapeutic agent. Another culture of control lobules was not infected and received no therapeutic agent. Parameters assayed were as follows: a) cell viability; b) p24 content; and c) CD4 + /8 + profiles. The results are shown in Table 6 and Figures 19-21. The data indicate that both ddl and AZT inhibit viral replication in the thymic lobule culture system, even when administered at sub-micromolar levels (Table 6) . AZT treatment was inhibitory even at extremely low doses (0.001-0.01 μM) , yet doses as high as 20 μM failed to completely block viral replication (Table 6 and Figures 19-21) . In contrast, ddl displayed a nearly linear inhibition of viral replication as drug doses increased above the 2 μM range, and complete blockage was seen at 20 μM (Table 6 and Figures 19-21) . A comparison between the percent reduction in p24 values seen at various doses of AZT and ddl for three separate experiments is shown in Table 7. The results presented in Table 7 suggest that the thymic lobule culture system can generate reproducible results reflecting the efficacy of both AZT and ddl in blocking HIV replication. Table 6 Doae rβaponse of ddl and AZT (Xho Strain) % Days Drug/ No. of Cells Per Reduction Abs.# Post- Virus Dose Frag¬ Fragment Percent in p24 % % % 4*8* Infection (XHO) ■ JJM) ments x 10 3 Live Cells Value CD4*8 CD48* CD4*8* x10 4 9 _ _, 14 211 33 100 21 11 61 7 9 + _ 14 111 14 o 24 25 32 2 9 + ddl 9 137 20 o 23 27 28 1 0.002 9 + ddl 13 149 18 23 24 21 36 3 0.020 9 + ddl 4 112 16 89 31 22 24 3 0.200 9 + ddl 2.000 4 108 24 96 28 18 37 4 9 + ddl 4 170 21 100 19 11 63 11 20.000 9 + AZT 5 107 16 22 16 27 37 2 0.001 9 + AZT 9 110 15 66 27 22 22 2 0.010 9 + AZT 4 99 23 82 26 21 24 2 0.100 9 + AZT 4 91 20 86 32 22 24 2 1.000 9 + AZT 9 112 22 91 25 18 46 4 10.000 9 + AZT 5 167 17 90 18 17 58 6 20.000 Table 7 Effect of ddl and AZT on Reduction of p24 Expression (Xho Strain) Percent Reduction in p24 Percent Reduction in p24 Percent Reduction in p24 Values as Compared to Controls Values as Compared to Controls Values as Compared to Controls Drug/Dose Experiment #1 Experiment #2 Experiment #3 (μM) (terminated at day 8) (terminated at day 9) (terminated at day 9) -none- 0% 0% 0% ddl 0.002 n.d. n.d. 0% ddl 0.020 n.d. 49% 0% ddl 0.200 82% 89% n.d. ddl 20.000 99% 96% n.d. ddl 20.000 100% 100% n.d. AZT 0.001 n.d. n.d. 22% AZT 0.010 n.d. 73% 54% AZT 0.100 89% 82% n.d. AZT 1.000 96% 86% n.d. AZT 10.000 93% 94% 87% AZT 20.000 n.d. n.d. 90% Two additional strains of HIV (EW and NL4-3) were also tested. The infection method, culture conditions, and controls were the same as described for the Xho strain. Percent reduction in p24 levels for various doses of AZT and ddl is shown in Figures 22 (EW strain) and 23 (NL4-3 strain) . Several infection parameters from these dose response experiments are summarized in Tables δa and 8b (EW strain) and Tables 9a and 9b (NL4-3 strain) . The results demonstrate that the thymic lobule culture system reflects the jLn vivo efficacy of both AZT and ddl in blocking replication of a variety of HIV strains. It was also found that the effects of ddl and AZT more closely approximate those seen in vivo (i.e., with SCID-hu Thy/Liv mice or humans) than in vitro (i.e., PHA-activated peripheral blood mononuclear cells (PBMC) ) . In contrast to the general observation in vitro f where ddl appears to be less effective than AZT in blocking HIV infection, both AZT and ddl appeared to have comparable efficacies in the thymic lobule culture system. These observations suggest that the thymic lobule culture assay system more closely parallels the in vivo SCID-hu Thy/Liv model than other in vitro cell assay systems for testing drug efficacy in blocking HIV infection. Table 8a Dose response of ddl and AZT (EW strain) Cells Per P 24 ratio Abs # % % AZT ddl No. of Fragment pg/10 4 p24%HIV CD cells 4+8 + % CD CD Day Virus μM μM lobes xlO 3 cells +no drug 4 to 8 %live xlO 6 CD4+ 4+8 + 8 + 9 NC 0 0 3 283 0 0 3.56 33 0.212 15.67 74.57 4.38 9 EW 0 0 3 207 21980 1 0.79 55 0.169 6.83 81.65 9.09 9 EW 0 0.01 3 73 34267 1.559 1.03 19 0.042 13.57 53.94 14.57 9 EW 0 0.1 3 186 4993 0.227 1.86 44 0.261 12.71 75.69 6.71 9 EW 0 1 3 413 409 0.019 2.97 44 0.343 10.52 82.78 3.56 9 EW 0 10 4 233 0 0 1.98 44 0.196 9.53 83.06 4.70 9 EW 0.01 0 3 140 22000 1.002 0.86 45 0.109 8.06 77.43 9.43 9 EW 0.1 0 3 280 2473 0.113 1.47 64 0.234 8.56 83.29 5.88 9 EW 1 0 3 387 1373 0.063 1.66 48 0.332 6.53 86.37 3.93 9 EW 10 0 3 173 8900 0.406 2.03 36 0.124 15.57 68.99 7.44 co to to Ul o l o * H Ul Ul Table 8b Dose response of ddl and AZT (EW strain) Cells p24 * ratio * Abs # * % * %' AZT ddl No. of Per * pg/10 5 p24%HIV CD cells * 4+8 + % * CD CD Day Virus μM μM lobes Fragment cells +no drug 4 to 8 %live xl0 β CD4+ 4 + 8 + 8 + ifxlO 5 9 NC 0 0 3 188 0 0 0.57 9 0.14 3.33 4.20 0.41 9 EW 0 0 3 12 3451 0.157 0.17 6 0.02 1.74 5.54 3.61 9 EW 0 0.01 3 23 6768.6 0.3078 0.37 14 0.03 2.28 18.68 6.13 9 EW 0 0.1 3 188 2815.1 0.128 0.16 3 0.16 6.08 10.07 2.89 9 EW 0 1 3 201 272.71 0.023 0.42 19 0.17 1.13 2.70 0.25 9 EW 0 10 4 82 0 0 0.34 25 0.09 4.68 21.40 6.20 9 EW 0.01 0 3 34 2828.4 0.128 0.66 19 0.19 3.93 19.09 6.81 9 EW 0.1 0 3 106 1651.7 0.075 0.71 18 0.10 2.07 2.87 0.38 9 EW 1 0 3 23 721.76 0.033 0.57 13 0.02 0.65 1.41 1.05 9 EW 10 0 3 100 4902.9 0.223 0.27 19 0.10 2.31 8.95 4.13 * These values represent standard deviations of corresponding rows in Table 8a. CO CO t to Ul o l o Ul Ul Table 9a Dose response of ddl and AZT (NL4-3 strain) Cells Per p24 ratio Abs -y % % AZT ddl No. of Fragment pg/10 6 p24 HIV CD cells 4+8 + % CD CD Day Virus μM μM lobes xlO 3 cells +no drug 4 to 8 %live xlO 6 CD4+ 4 + 8+ 8 + 8 NC 0 0 4 141 0 0 1.68 36 0.117 9.62 81.53 5.57 8 NL4-3 0 0 5 * 36 24644 1 0.80 14 0.016 17.40 42.69 21.84 8 NL4-3 0 0.1 5 116 5001 0.203 2.01 36 0.081 16.25 69.93 8.31 8 NL4-3 0 1 5 142 907 0.037 1.50 49 0.119 7.79 83.96 5.21 8 NL4-3 0 10 5 102 61 0.002 2.41 42 0.079 13.60 77.94 5.53 8 NL4-3 0 100 5 195 17 0 3.13 42 0.153 13.92 78.33 4.57 8 NL4-3 0.1 0 5 72 1732 0.071 1.59 41 0.058 10.44 80.05 6.81 8 NL4-3 1 0 5 179 1007 0.041 1.85 52 0.153 8.60 84.57 4.56 8 NL4-3 10 0 5 142 978 0.041 2.36 35 0.111 13.36 77.32 5.72 8 NL4-3 100 0 5 83 1627 0.066 1.77 24 0.051 22.85 58.97 12.89 'Except p24 data is only 3 lobes. CO CO to to Ul o Ul o H Ul o Ul Table 9b Dose response of ddl and AZT (NL4-3 strain) Cells p24" ratio * Abs # * %' %' AZT ddl No. of Per * pg/10 6 p24%HI * CD cells * 4+8+ %' CD CD Day Virus μM μM lobes Fragment cells V 4 to 8 %live xlO 6 CD4+ 4 + 8 + 8 + xlO 5 +no drug 8 NC 0 0 4 55 0 0.00 0.49 6 0.05 4.34 5.82 0.89 8 NL4-3 0 0 5 * 12 7377 0.30 0.08 3 0.01 3.67 13.85 4.17 8 NL4-3 0 0.1 5 63 415 0.02 0.52 6 0.04 2.53 2.57 1.44 8 NL4-3 0 1 5 18 343 0.01 0.05 8 0.02 0.45 1.51 0.40 8 NL4-3 0 10 5 18 35 0.00 1.05 7 0.02 6.50 7.94 0.42 8 NL4-3 0 100 5 102 15 0.00 0.88 3 0.08 1.86 0.97 0.72 8 NL4-3 0.1 0' 5 39 369 0.02 0.33 15 0.03 0.39 3.03 1.87 8 NL4-3 1 0 5 47 181 0.01 0.42 7 0.05 3.03 3.81 0.86 8 N 4-3 10 0 5 94 333 0.01 0.32 2 0.07 0.54 0.44 0.63 8 NL4-3 100 0 5 62 1323 0.05 0.12 1 0.04 1.90 1.88 0.22 'Except p24 data is only 3 lobes. These values represent standard deviations of corresponding rows in Table 9a. b. Effects of Various Inhibitors This experiment describes the ability of various cell signalling inhibitors, including protein tyrosine kinase, protein kinase C and protein phosphatase inhibitors to inhibit viral infection. These various protein inhibitors have different modes of action as indicated in Table 10 below. The results from each of these inhibitors are also summarized below. Table 10 Compound Source Mode of Action Comments Herbimycin Calbiochem Protein tyrosine- inase inhibitor Tautomycin Calbiochem Protein phosphatase does not activate PKC inhibitor Bisindolylmaleimide Calbiochem Protein Kinase C inhibitor highly selective inhibitor of ATP-binding site of PKC H-7, dihydrochloride Calbiochem potent inhibitor of PKC and cyclic-nucleotide-dependeπt protein kinases 1. Herbimycin A Given the broad range of parameters of HIV- associated pathology that can be assessed in the TOC system, we have analyzed the effect of herbimycin A (a broad range protein tyrosine kinase inhibitor) in TOC following infection with the HIV primary isolate JD. Human fetal thymus fragments were infected with JD, a primary isolate of HIV. At two days post-infection, normal medium was replaced with medium containing either 1 μM or 4 μM herbimycin A. Medium with drug was replaced every day thereafter until the termination of the experiment. Fragments were sacrificed at days 4 and 7 post-infection, and thymocytes were analyzed by flow cytometry for CD4/CD8 expression, MHC Class I expression, TdT reactivity, and by p24 ELISA. The results of this experiment are given below in Table 11 and Figure 24. O ω to t Ul o Ul o Ul Ul Table 11 Summary of Herbimycin A Effects on JD Infection in TOC. Analysis of Cell Recovery, CD4/CD8 Profile, and p24 Values Following Infection with JD in TOC herbi¬ days in number of cells percent percent percent percent CD4:CD8 p24 Virus mycin A culture fragments x IO 3 live gate CD4 + CD8 + CD4 + 8 + ratio pg/10 6 cells none 0 4 4 1303 40 7.1 4.1 87 1.74 0 none lμM 4 4 1307 54 4.6 3.2 91 1.44 0 none 4μM 4 4 1157 44 5.6 3.5 90 1.67 0 JD 0 4 6 494 32 5.3 5.1 88 1.07 697 . JD lμM 4 5 665 21 5.9 4.2 89 1.42 250 JD 4μM 4 6 675 34 5.5 4.5 89 1.23 1071 none 0 7 6 288 38 14.1 8.6 75 1.65 0 none lμM 7 6 288 41 10.2 6.5 82 1.58 0 none 4μM 7 6 243 16 18.4 8.6 70 2.13 0 JD 0 7 8 45 2 8.5 40.9 30 0.21 191900 JD lμM 7 8 63 3 9.1 44.7 29 0.20 193900 JD 4μM 7 7 63 1 4.6 39.3 45 0.12 267200 In the case of uninfected fragments, herbimycin appears to have little effect on cell recovery and CD4/CD8 profiles. Possibly, the modest decrease in cell recovery, CD4 + 8 + percentage and the more pronounced reduction in percent live gated events at day 7 of culture for the high dose of herbimycin A reflect some toxicity of the drug in TOC. In the case in infected fragments, herbimycin A does not elicit dramatic effects in comparison to the untreated group when looking at cell recovery, percent live gated events, and CD4/CD8 profiles. Similarly, the upregulation of MHC Class I in each of the three infected groups shows identical levels of expression, despite varying concentrations of herbimycin A (Figure 24) . In sharp contrast, examination of TdT reactivity at day 4 post-infection demonstrates very interesting effects within the infected group of fragments (Figure 24) . Each of the uninfected groups (no drug, 1 μM herbimycin, 4 μM herbimycin) exhibits similar levels of TdT reactivity, suggesting that for uninfected thymocytes, two days treatment with herbimycin A has little effect on endogenous levels of DNA breakage. Untreated fragments which were infected with JD show a market increase in TdT reactivity by day 4 in culture. When 1 μM herbimycin A was added to JD-infected TOC, at day 4 the level of TdT reactivity is maintained at the level seen for uninfected TOC. When 4 μM herbimycin A was added to JD-infected TOC, at day 4 the level of TdT reactivity was reduced below the level observed for uninfected controls. At day 7 post-infection, TdT analyses were less informative as JD-induced thymocytes depletion had progressed very far. Furthermore, it appears that longer culture in the presence of herbimycin A, particularly at the higher concentration, resulted in increased TDT reactivity, even for the uninfected group. Based upon the results, it appears that the protein tyrosine kinase inhibitor herbimycin A does not inhibit HIV-induced MHC Class I upregulation when added at two days post-infection. However, the drug does appear to inhibit DNA breakage at early timepoints after infection. This data suggests a role for tyrosine phosphorylation in HIV-induced programmed cell death. These results also imply that various drugs might be effectively studied for efficacy in TOC using one or several of the readouts currently available. The use of drugs with greater specificity may help delineate specific signalling pathways by which HIV triggers cell death in thymocytes. A number of such reagents have been tested in TOC and are reported below. 2. Tautomvcin At 20 nM, administered either on day 2, or days 2, 3 and 4, this compound exhibited possible toxicity on uninfected TOC with respect to a slight decline in total cell recovery and in the percent of live cells on a per fragment basis. Moreover, this compound generated a slight increase in DNA strand breakage, as measured by the TdT-FACS assay. This drug did not appear to effect viral replication, nor virus-induced pathology at the dose administered. 3. Bisondolylmaleimide In the first experiment, this compound was administered at 40 or 200 nM on days 3 or days 3 and 4 of culture, with the experiment being terminated on day 5. At these doses the drug did not appear to be toxic with respect to the total cell recovery and in the percent of live cells on a per fragment basis, except in the case of the highest dose administered to uninfected TOC. In this instance, a slight decrease in fragment cellularity was observed. While the compound did not appear to effect CD4 and CD8 distribution patterns, nor Class I expression profiles for uninfected TOC, at the highest dose in infected TOC, CD4:CD8 ratios were seen to return to a normal profile. In the case of uninfected TOC, Class I expression remained high at all doses examined. Culture of infected TOC in the presence of this inhibitor resulted in a dose-dependent reduction in virus replication and an increase in the number of cells recovered per fragment (low dose=25% reduction in p24; high dose=50% reduction in p24) . This compound also generated a dose-dependent reduction in DNA strand breakage for infected TOC as measure by TdT-FACS. At the lower dose, there was an approximate 30% reduction in TdT-positive cells, and at the higher dose, the reduction was near 85% over that seen for infected and untreated TOC. In the second experiment, bisondolylmaleimide was administered at 40 of 200 nM on days 3 and 4 and similar results to the first experiment were obtained with minor differences. In comparison to infected/untreated TOC, the reduction in p24 observed in this experiment was 24% at the low dose, and 60% at the high dose. At the highest dose, cell yields on a per fragment basis were improved to the level of uninfected controls. As before, the highest dose of drug improved CD4:CD8 ratios. In contrast to the first experiment, low dose of drug led to a 10% reduction in TdT-positive cells, while high doses resulted in a 65% reduction. 4. H-7 dihydrochloride This compound was administered at 20 or lOOμM on day 3 or days 3 and 4 of culture, with the experiment being terminated on day 5. H-7 appeared to exhibit some toxicity at the highest dose as demonstrated by a reduction in cell recovery and the percent of live cells on a per fragment basis in both uninfected and infected TOC. Interestingly, this toxicity was most apparent in the reduced representation of the mature CD4 + and CD8 + thymocytes. For either uninfected or infected TOC, the drug did not appear to affect CD4 + 8 + , or progenitor CD4 + 3 + thymocytes. At 20 μM, H-7 appeared to inhibit viral replication, based upon a reduction in p24 values. At 100 μM, the compound completely blocked viral replication, as measured within the sensitivity of our standard p24 ELISA assay. Moreover, H-7 reduced MHC Class I expression levels in both control and infected cultures. TdT-FACS data from this experiment was inconclusive. These results suggest a possible correlation between specific toxicity of the drug on mature thymocytes and the drug's ability to inhibit viral replication. Example 7 Biochemical Analysis of Cell-Cvcle Regulatory Proteins in TOC Following Infection with HIV In order to decipher the events which lead to HIV-mediated thymocyte death, we have begun to characterize the effects of HIV infection on various cell-cycle regulatory pathways in both TOC and in SCID-hu Thy/Liv mice. The tightly regulated interactions of the various cyclins, their specific kinases and phosphotases, and associated cytoplasmic or nuclear inhibitors are all essential components in governing appropriate progression through the cell cycle. Dysfunction of any or all of these elements can lead to cell-cycle arrest and in some cases cell death. Such cell cycle dysfunction has been suggested to be involved in HIV pathogenesis in both in vitro cellular assays and in SCID-hu Thy/Liv mice. In the experiments described below, we examine the expression levels of cyclin Bl from infected and uninfected TOC and SCID-hu Thy/Liv mice by Western. Figure 25 shows the structure of cyclin Bl. The expression of this protein is regulated at the transcriptional level and at the proteolytic level. As cells progress into mitosis, cyclin Bl's "destruction box" targets the protein for ubiquitination followed by rapid proteolysis. This proteolysis is thought to be required, in part, for the inactivation of the cyclin Bl/p34 cc2 complex, a process essential for normal cell division and survival. As seen in Figures 26(a) and (b) , we have now observed the accumulation of cyclin Bl proteolytic fragments in thymocytes following infection in both TOC and in SCID-hu Thy/Liv mice. Western analysis of cyclin Bl levels in infected and uninfected TOC lysates generated very interesting results (Figure 26(a)). While little change is seen in the relative levels of the natural 62 kD cyclin Bl protein in uninfected vs. infected lysates, there appears to be a significant accumulation of a 18.5 kD protein moiety and, to a lesser extent, a 14.5 kD protein moiety, in lysates from infected TOC. In addition, an accumulation of 35 Kd and 53 Kd proteins are seen when a rabbit polyclonal antiserum is used for detection of p34cdc2. Since the deregulation and turnover of cyclin Bl must be tightly regulated for a cell to avoid mitotic catastrophe, the potential HIV- • associated accumulation of cyclin Bl or a related by¬ product could be a key element underlying HIV's role in leading to thymocyte death. Whether these species are proteolytic fragments of the natural cyclin Bl or represent a cross-reactive protein species, remains to be determined. Cytokine Profiles a. Changes in cytokine profiles following HIV infection have been compared in SCID-hu Thy/Liv and in TOC. Thymocytes from Thy/Liv or from TOC were lysed and lysates analyzed by ELISA. Preliminary results are given below in Table 12 for IL4, IL6, IL10 and TNFα. Table 12 Determination of cytokine profiles in TOC and SCID-hu Thv/Liv Mice following infection with HIV Using an ELISA-based analysis system, we have assessed expression profiles for a number of cytokines thought to be associated directly or indirectly with HIV pathogenesis. We have observed the following: TOC SCID-hu Thv/Liv IL-4 Increased expression Increased expression (in 3 of 4 experiments) (in 2 of 2 experiments) IL-6 Increased expression Increased expression (in 4 of 4 experiments) (in 2 of 2 experiments) IL-10 Increased expression Increased expression in (in 5 of 6 experiments) 2 of 2 experiments for infection with JD and slight decrease in 1 experiment for infection with NL4.3 TNF-α Increased expression Increased expression (2 of 3 experiments) (in 2 of 2 experiments) While these results are only preliminary, the upregulation of TNFα, IL6 and IL4 in the SCID-hu model and the increase in IL10 production in TOC following infection with HIV parallels trends observed in HIV- infected individuals. Tyrosine Phosphorylation Cell signalling profiles have been studied during HIV infection in TOC and in SCID-hu Thy/Liv mice. In the experiment described below, we infected TOC with either the molecular isolate NL4.3 or with the clinical isolate JD. TOC were sacrificed at days 4, 5 or 7. Cytoplasmic lysates were made from infected and uninfected TOC and these lysates were subsequently analyzed by WESTERN blot for infection-associated tyrosine phosphorylation events. Blots were probed with a monoclonal antibody specific for phosphorylated tyrosine residues. Lysates from uninfected and JD- infected day 4 TOC were compared to each other. Similarly, lysates from uninfected, NL4.3-infected, and JD-infected day 5 TOC, or uninfected and NL4.3-infected day 7 TOC were compared to each other. While overall tyrosine phosphorylation patterns were similar for all of the lysates, some individual proteins demonstrated enhanced phosphorylation following infection with HIV. The size and approximate increase in phosphorylation level for the bands are indicated below in Table 13. While the exact sizing of these bands awaits more careful determination, there are clearly a number of proteins that appear in the cytoplasmic lysates of infected TOC that are phosphorylated at tyrosine, and which are absent, or present in lower amounts in uninfected cultures. For some of these bands, particularly the lower molecular weight species, there appears to be more protein present in infected TOC than in uninfected TOC based upon Coomassie staining of post- transfer gels. Whether the increase in observed phosphorylation is due to hyperphosphorylation or a simple increase in protein levels has yet to be determined. Table 13 Virus days in cells per 14KD 20KD 23KD 34KD 40KD 105KD culture lane (xlO 5 ) band band band band band band JD 4 5 + + + + + + + + + + + JD 5 5 + + + + + + + nd nd nd NL4.3 5 5 + + + nd nd nd NL4.3 7 5 + + + + + + + + + + nd nd For different virus and different timepoints, the relative increase in intensity of bands over those seen in uninfected controls as detected by the antiphosphotyrosine monoclonal antibody by WESTERN. + = slightly higher intensity of band ++ = moderate increase in intensity of band +++ = large increase in intensity of band

Claims: Clai s 1. A method of identifying potential anti¬ viral agents comprising assessing the ability of the agents to suppress viral replication and/or pathology in an in vitro thymic organ culture. 2. A method of screening agents for potential anti-viral efficacy comprising the steps of: a. providing thymic lobules; b. incubating the lobules for a time and under conditions sufficient to allow viral infection; c. exposing the lobules to a virus under conditions sufficient to induce infection; d. exposing the lobules to an agent to be screened; and e. assaying cells from the lobules for viral activity; wherein the agent is considered a potential anti-viral agent if viral replication or pathology is suppressed relative to the cultured lobules in a control sample. 3. The method according to claim 2 wherein the thymic lobules are human. 4. The method according to claim 2 wherein the thymic lobules are human fetal. 5. The method according to claim 2 wherein the thymic lobules are human neonate. 6. The method according to claim 2 wherein the thymic lobules are human adolescent or adult. 7. The method according to claim 2 wherein the thymic lobules are contained in about 1 mm 3 fragments containing 1-4 intact lobules. 8. The method according to claim 2 wherein the initial incubation step of the thymic organ culture is for about 6-10 days. 9. The method according to claim 2 wherein the lobules are infected by incubation in a liquid containing the virus particles for a length of time suitable to allow infection. 10. The method according to claim 9 wherein the lobules are incubated in the liquid for approximately 2 hours. 11. The method according to claim 9 wherein the lobules are continuously rocked while they are incubated in the liquid. 12. The method according to claim 2 wherein the lobules are infected by dripping virus-containing liquid over the lobules. 13. The method according to claim 2 wherein the virus used for infection is selected from the group consisting of human cytomegalovirus, human immunodeficiency virus, herpes simplex virus, and rubella virus. 14. The method according to claim 2 wherein the initial incubation step is performed in an atmosphere containing about 40 to 60% oxygen. 15. The method according to claim 14 wherein the atmosphere contains about 50% oxygen. 16. A method of screening agents for potential anti-viral efficacy comprising the steps of: a. incubating human fetal thymic lobules for a time and under conditions sufficient to allow viral infection; b. exposing the lobules to a virus under conditions suitable to allow infection of the lobules by the virus; c. exposing the lobules to an agent to be screened; d. assaying cells from the lobules for viral activity; wherein the agent is a potential anti-viral agent if viral replication or pathology is suppressed relative to a control sample. 17. The method according to claim 16 wherein the initial incubating step is for about 6-10 days. 18. The method according to claim 16 wherein the lobules are infected by incubation in a liquid containing the virus particles for a length of time suitable to allow infection. 19. The method according to claim 18 wherein the cells are incubated in the liquid for approximately 2 hours. 20. The method according to claim 18 wherein the lobules are continuously rocked while they are incubated in the liquid. 21. The method according to claim 16 wherein the cells are infected by dripping virus-containing liquid over the lobules. i 22. The method according to claim 16 wherein the virus used for infection is selected from the group consisting of human cytomegalovirus, human immunodeficiency virus, herpes simplex virus, and rubella virus. 23. The method according to claim 16 wherein the initial incubation step is performed in an atmosphere containing about 40 to 60% oxygen. 24. The method according to claim 23 wherein the atmosphere contains about 50% oxygen. 25. A method of identifying potential therapeutically effective agents for the treatment of human immunodeficiency virus infection comprising the steps of: a. providing a culture of human fetal thymic lobules; b. incubating the culture under conditions suitable to maintain cell viability; c. exposing the lobule culture to human immunodeficiency virus under conditions suitable to induce infection of the lobules by the virus; d. exposing the lobule culture to an agent to be screened; and e. assaying the cells from the lobules for human immunodeficiency virus activity; wherein the agent is determined to have potential therapeutic efficacy if virus replication and/or pathology is suppressed relative to lobules in a control sample. 26. The method according to claim 25 wherein the initial incubation step is for about 6-10 days. 27. The method according to claim 25 wherein the lobules are infected by incubation in a liquid containing the virus particles for a length of time suitable to allow infection. 28. The method according to claim 25 wherein the lobules are incubated in the liquid for approximately 2 hours. 29. The method according to claim 25 wherein the lobules are infected by dripping virus-containing liquid over the cultured lobules. 30. A method of testing hypotheses about mechanisms of viral infection and pathology comprising a. providing a culture of thymic lobules; b. incubating the culture under conditions suitable to maintain cell viability; c. exposing the lobules culture to a virus capable of infecting the lobules; d. observing the cells from the lobules for viral activity; and e. comparing the observations of step d with the hypothetical mechanism. 31. The method according to claim 30 wherein the thymic lobules are human. 32. The method according to claim 30 wherein the thymic lobules are human fetal. 33. The method according to claim 30 wherein the thymic lobules are human neonate. 34. The method according to claim 30 wherein the thymic lobules are human adolescent or adult. 35. The method according to claim 30 wherein the lobules are contained in about 1 mm 3 fragments containing 1-4 intact lobules. 36. The method according to claim 30 wherein the initial culturing step is for about 6-10 days. 37. The method according to claim 30 wherein the lobules are infected by incubation in a liquid containing the virus particles for a length of time suitable to allow infection. 38. The method according to claim 37 wherein the lobules are incubated in the liquid for approximately 2 hours. 39. The method according to claim 37 wherein the lobules are infected by dripping virus-containing liquid over the lobules. 40. The method according to claim 30 wherein the virus used for infection is selected from the group consisting of human cytomegalovirus, human immunodeficiency virus, herpes simplex virus, and rubella virus.

Inventors: BONYHADI MARK L, KANESHIMA HIDETO, MCCUNE JOSEPH M, BONYHADI, MARK, L., KANESHIMA, HIDETO, MCCUNE, JOSEPH, M.
Applicants: BONYHADI MARK L, KANESHIMA HIDETO, MCCUNE JOSEPH M, SYSTEMIX INC, BONYHADI, MARK, L., KANESHIMA, HIDETO, MCCUNE, JOSEPH, M., SYSTEMIX, INC.
Application year: 1994
Main classification: C12N5
IPC codes: C12Q1, G01N33